MPI_File_iread is a nonblocking version of MPI_File_read. It
attempts to read from the file associated with fh at the current individual
file pointer position maintained by the system in which a total number
of count data items having datatype type are read into the user’s buffer
buf. The data is taken out of those parts of the file specified by the
current view. MPI_File_iread stores the number of data-type elements actually
read in status. All other fields of status are undefined. It is erroneous
to call this function if MPI_MODE_SEQUENTIAL mode was specified when the
file was opened.

Almost all MPI routines return an error value;
C routines as the value of the function and Fortran routines in the last
argument. C++ functions do not return errors. If the default error handler
is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception
mechanism will be used to throw an MPI::Exception object.

Before the error
value is returned, the current MPI error handler is called. For MPI I/O
function errors, the default error handler is set to MPI_ERRORS_RETURN.
The error handler may be changed with MPI_File_set_errhandler; the predefined
error handler MPI_ERRORS_ARE_FATAL may be used to make I/O errors fatal.
Note that MPI does not guarantee that an MPI program can continue past
an error.